1. Field of the Invention
The invention relates to a method and arrangement for attenuating mechanical resonance in a loudspeaker.
2. Description of the Relevant Art
For about 50 years, it has been known in the art that mechanical resonances in a loudspeaker decrease the quality of emitted sound, and various methods have been devised to overcome the problem. A plurality of these draw upon elastically mounting the loudspeaker element to the enclosure, thus attenuating the transmission of vibrations to the enclosure. However, such structures proposed in many different embodiments are difficult to implement, expensive to produce and require the design and manufacture of nonstandard means of mounting the loudspeaker element.
Thomasen [1] and [5] have patented a vibration damper suitable for attachment to the enclosure wall, with the aim of reducing vibrations in enclosure walls. Because this method does not attempt to reduce the exciting vibration at its source, but rather, the secondary effects of wall motion, it fails to provide an effective approach to control vibration. Furthermore, it is a general object in the art to control vibrations over a wide bandwidth, whereby it can be shown that the disclosed embodiment is incapable within the constraints of practicable designs and materials to combine wide-band operation with high efficiency.
Akroyd [2] presents a construction wherein a tube made of an elastic material couples mechanically the dynamic loudspeaker element to the enclosure wall. This arrangement aims to support the frame of the loudspeaker element to reduce vibrations. At the same time, the tube acts as an acoustical resonant structure. While providing mechanical support, the structure fails to act as an efficient attenuator of enclosure resonances. The invention claims that this mechanical coupling achieves cancellation of vibrations, and thereby a reduction thereof. It is known that as the stiffness of material can be increased, e.g., by additional supporting, the eigenfrequencies of characteristic resonances increase, but the resonances are not removed unless the frictional losses of the structure that cause attenuation of resonances cannot be simultaneously increased. In fact, if the driver unit of a dynamic loudspeaker element is coupled mechanically to the rear wall of an enclosure, the amount of mechanical vibrations in the external walls of the loudspeaker enclosure are actually likely to increase rather than to decrease, because this construction actually enhances the mechanical coupling of vibrations at most frequencies, instead of reducing the coupling. However, one effect of the additional mechanical support is to move the eigenfrequencies of mechanical resonances to higher frequencies.
Tanaka [3] presents a construction where the driver unit of a dynamic loudspeaker element is fixed to some external part of the loudspeaker enclosure other than its front wall. Additionally, there is provided an elastic means of fixing the frame of the loudspeaker element to the loudspeaker enclosure. The comments expressed about reference [2] apply equally well to this invention, because again there is provided a construction wherein the loudspeaker driver unit is mechanically attached to the enclosure using means having low mechanical losses, although herein the fixing point is not on the front of the enclosure. Because vibrations that are coupled to the enclosure walls normally occur on all walls of the enclosure, this invention will not lead to a good end result. No matter to which wall the driver unit of the loudspeaker element has been attached, mechanical vibrations will appear in all walls of the enclosure, and the transmission of mechanical energy to the enclosure becomes particularly efficient at a frequency where the mechanical elasticity of the attachment means and the mass of the loudspeaker element will resonate. Therefore, the invention cannot generally reduce the coupling of vibrations to the enclosure, although it may marginally reduce vibrations that are coupled to the front wall of the enclosure.
The invention disclosed by Favali [4] presents a loudspeaker enclosure with a construction that aims to attenuate mechanical vibrations by using plates made of an elastomer that bond together the walls of the enclosure and attach the loudspeaker element to the enclosure. The goal herein is to create shear forces into the elastomer that serves to convert mechanical energy into heat by internal friction in the material. This invention does not attempt to reduce the tendency of the loudspeaker element to cause mechanical vibrations. The structure is not efficient at resonant frequencies whose maximum displacement does not occur at the elastomer joints because there is no acoustic energy loss at these frequencies in the elastomer material.
It is typical of the prior-art solutions [1-5] that there is no attempt to control the mechanical vibration at its source, i.e., at the loudspeaker element, but rather they pursue to affect the secondary vibrations in the loudspeaker enclosure.
The present invention differs from the prior art in that it is a particular object of the invention to attenuate the mechanical vibration of the loudspeaker element driver unit, thereby making it unnecessary to attenuate vibrations in the enclosure structures. In this way, the present invention is different from and already basically superior to conventional constructions.
The goal of the invention is attained by elastically attaching at least one additional mass to the magnet circuit of the loudspeaker, with the masses chosen such that the eigenfrequencies of the system will typically coincide with the mechanical resonances of the loudspeaker. With this provision, the mechanical vibration energy produced by the magnet circuit tends to become transferred to the additional masses as a vibration of the additional masses, thus allowing the elastic coupling elements to absorb this energy by the frictional losses of the material. Typically, the total additional mass is chosen to be of the same order of magnitude as the mass of the magnet circuit. The masses may also differ by their order of magnitude from the mass of the magnet circuit.
The invention offers substantial benefits.
Control of resonance attenuation by the virtue of the present method is cheaper to implement than by using the prior-art techniques, because it is not necessary to modify the good and well-proven principles of loudspeaker construction in order to remove undesirable resonances. This is not possible if the loudspeaker element is attached to the enclosure using elastic means, if the magnet circuit is elastically attached to the frame of the loudspeaker element, or when using elastic structures in the loudspeaker enclosure. Furthermore, by a proper choice of the additional masses and the elasticity and losses in their attachment, it is possible to adjust the Q-value of the resonance peaks, the effective frequency range of control and the amount of vibration reduction.